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E-beam pumped GaInP/AlGaInP MQW VCSEL.

V.Yu. Bondarev l), V.I. Kozlovsky I) , A.B. Krysa z), J.S. Roberts '1, Ya.K. Skasyrsky 1IP.N. Lebedev Pli~.stcol Irariare of RAS. 53 L e n i n s b p ~ . 119991 Moscow, Riirsia 2)Tke U n i ~ w s i ~ of Sl&ield EPSRC Norioriol C e n w fur Ill-V Technologies, UK

Abstract

A 17-period G ~ . s I n o . ~ P / ( A l o , ~ G a ~ , ~ ) ~ , ~ I n ~ . s P quantum well structure was grown by metalorganic vapor phase epitaxy on a GaAs substrate misoriented by IOo from (001) to (111)A. A microcavity with dielecmc oxide mirrors was fabricated on the basis of this sVucNre. Lasing at 619 nm with 0.7 W output power was achieved under scanning electron beam longitudinal pumping at room temperature. It is shown that low threshold lasing requires the position of the QWs to coincide with the antinodes of the cavity mode, at the maximum of the gain spectrum due to the QW ground state.

I. Introduction

Laser cathode ray tubes (LCRT) emitting in red, green and blue spectral ranges are potentially imponant candidates for displays.('.') To date the best characteristics of LCRT have been obtained with laser active elements or laser screens (LS) made from 11-VI hulk single crystals. However the laser threshold for room temperature operation remains too high for commer- cial applications. It has therefore been proposed that MQW

was designed such that the QWs were located at the antinodes of a laser mode (Fig. 2). were the barrier thickness is smaller than double carrier diffusion length.

The cavity was pumped by a scanning electron beam with energy E, = 25-50 keV and current I , = 0-1.7 mA with an e- beam diameter de = 20-50 pm, depending upon the electron energy and current. The scan velocity was about 4 x lo5 c d s . The repetition rate of the scan was 50 Hz.

._________________- lasers would have lower thresholds and higher output powers. ,- 6 m G W Previously, MBE grown ZnCdSeEnSe MQW lasers with scan- ning e-beam longitudinal oumnine fanaloeue of VCSEL with L " L . - , - injection pumping) had been realized in the blue-green spectral r a n ~ e . ( ~ - ~ ) I 286.5 mn AlGalnP - 17 bm.m I

6 n m W

1 (M G& -buff=

In this paper we present main characteristics of the first red emitting e-beam pumped VCSEL made from G~,SInu.sP/(Alo.-iGso3)o.slno.sP MQW 8UUcNns grown by

11. Experimental details e- -beam

\ I / The MQW structure was grown by MOVPE on GaAs sub- 111

MQW excitation area

438pUnwOaInP 6nmGaInP

6 pair S i W i Q minx

sspphue

sirate misoriented by 10' from (001) to (111)A. The structure contains a 1 fim thick GaAs buffer layer, 6 nm of GaInP and 17 pairs of 286.5 nm AlGaInP banier material with 8 nm thick GaInP QWs. The etalon was completed by a 4.38 bm AlGalnP passive layer, capped by 6 nm of GaInP. The 6 nm layers of

idizing during cavity fabrication. Six pairs of quarter wave S i 0 2 and Ti02 layers were coated

on the structure to form first mirror of the cavity. This s m - ple was then glued by epoxy to a sapphire holder and the GaAs substrate and buffer removed by polishing, followed by etching in KOH-NH30H-H2O?-H?O solution. A second 7 pair mir- ror of the same structure was then coated on the etched surface to complete the eralon. The structure and the laser etalon are sketched in Fig. 1. The configuration of the structure and etalon

GaInP layers were used to prevent the AlGaInP layers from ox- epoxy

Fig, I Sketches of as-grown structure and laser etalon.

111. Results and discussion

The top surface of the grown structure showed good mor- phology with an RMS roughness of less than 0.6 nm measured

0-7803-7704-4/03/$17.00@2003 IEEE 182

by AFM although the total structure thickness with the GaAs buffer layer was as great as 10.4 pm. The AFM pattem of the structure surface is presented in Fig. 3. Observed features are likely to be steps of the growth.

I l l * 1 1 1

Fig. 2 Sketch of QW position with respect to cavity mode.

The room temperature cathodoluminescence (CL) spectrum of this svuctux is shown in Fig. 4 with a peak emission at 630

Lasing was achieved at electron energies from 25 to 50 keV and T = 80 K and 300 K (RT). At RT the threshold current was as low as 25 mA at 40 keV, corresponding to current density& = 8 A/cm2. This value is 7 times smaller than the typical threshold of LS made using bulk CdSSe (620 nm) and the same electron energy. The output power of the device was 0.7 W at RT, us- ing E, = 40 keV and I , = 1.7 mA. However, this output is about three times smaller than that of a similar hulk II-VI laser device. The total divergence angle did not exceed 15' for the maximum e-beam current used. The far field pattem of the laser output is presented in Fig. 5 . The laser emission was strong polarized with electric field vector parallel to the < 110 > direction. Fig. 4 also presents the laser emission specmm for 1.7 mA e-heam current 0'. = 70 A/cm2). Above threshold the ldsing specuum contains only one longitudinal cavity mode at 619 nm. Funher- more, the spectral mode position was found to be coincident with minimum of the PR spectrum which is on the shon wave- length side of QW emission line.

8 E-beam pumwa GalnPlAlGalnP MOW VCSEL

Wavelength (nm)

Fig. 4 CL at E , = 30 keV. j. = 0.1 mA/cm2 (lower curve) and PR (upper curve) spectra of as-grown StNC- ture, and lasing spectrum of VCSEL emission at E, = 40 keV, j e = 70 A/cm2.

Fig. 3 AFM pattem of as-grown structure surface.

nm. Fig. 4 also shows the photoreflection (PR) spectrum. with a modulation arising from Fabry-Pemt effects from the total structure thickness together with a clear feature near 620 nm due to the MQW period.

Fig. 5 Far field pattem of the laser emission with 0.7 W output at RT.

For T = EO K the lasing wavelength was shifted to 613 nm, close to the minimum of the PR spectrum at this temperature. However, at this temperature lasing takes places on long wave side of the 610 nm spontaneous emission line. Consequently,

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the threshold current was much lower at only 1.2-1.5 /LA at 40 keV, corresponding to current density j , < 0.5 Alcm’. The out- put power versus the e-beam current is presented in Fig. 6 at

E-beam wmnl (mA)

Fig. 6 The output power vs. e-heam current at electron energy of 40 keV and two temperatures: T = 80 K and RT.

both RT and T = 80 K. The maximum output was 2.3 W at 80K with an efficiency of 0.036. Under these conditions two or three longitudinal modes were generated with an inter-mode distance of 4.2 nm.

Following an extended electron beam exposure of the laser screen at low temperature the mirror was seen to lift from the etalon as a result of poor bonding. Consequently, there was a change of the mirror reflection phase and mismatching of the QW position with respect to the cavity mode antinodes. The to- tal gain per cavity pass for the ground state QW levels is there- fore unable to compensate for the total cavity losses. Neverthe- less, lasing was achieved at a higher e-beam intensity using gain derived from the fist excited levels in the QWs. In this case the lasing appears in 570-580 nm spectral range.

IV. Laser model

The condition for lasing threshold can be writen

G(E)Arejr = L o s - 0.51n(R1R:!) (1)

where G(EJ is linear gain per QW. N,ff is the effective number of participating QWs, h is the internal losses per cavity pass, RI and R:! are the front and hack mirror reflectivities , respec- tively. Ne, , depends on position of the QWs inside the cavity(’)

( 2 )

wherek, = (2m-dl -d2)/2L,isaneigenvectorofthecav- ity mode, L, is the cavity length. d, are the mirror phases and the zi are the positions of the quantum wells relative to the back mirror boundary. Dependence of N,ff on wavelength of cavity modes is presented in Fig. la. At that the values of L,, Lb + L, were corrected taking into consideration the real period of the StmCNre obtained from the reflection spectrunl and oi was cal- culated for used nlirror design. Refractive indices for dielectric

A‘,,, = CC062(k”,(Z, - L) - d2)

layers of the mirrors were taken such that the calculated reflec- tion spectrum of the mirror was the same as the measured one. One can see the maximum value of Ne, , is 15 that is smaller than the number of QWs in the structure. It means that the po- sitions of the QWs is not exactly matched with antinodes of the cavity mode.

The linear gain G is calculated, using the following equation

where E. EO, G,,,f,,ft, and L(E,E’) are optical transition energy considering, base emission energy for QW, gain constant which is dependent on the QW material and the QW structure, electron and hole Fed-distribution functions, and line shape function. respectively.

2-D electron density is calculated by using the following equation

where Lb. nz:, in:, k ~ , T. R, E;, AEc , and E, are barrier thick- ness, elecuon effective mass in the well and the harrier, Boltz- mann’s constant, temperature. Planck‘s constant divided by 2 71, electron Fermi energy, conduction band discontinuity between the QW and the harrier. and the electron i-suhband edge enersy, respectivity. The energy base is the QW conduction band edge.

The 2-D hole density is calculated in the following

where indices I , h mean light and heavy holes. I ~ I ; ~ , m:,b. 9, and AE, are the hole effective mass in the well and the bar-

rier. hole Fermi energy, the hole j-subhand edge energy and the valence energy separation between the QW and the barrier, re- spectively. Energies are measured from the QW valence-hand edge in this case, and their values are taken to be positive.

The gain constant Go in (3) is described as follows

where /LO, :.CO, L,,, e, Eg and A are vacuum magnetic perme- ability, dielectric constant for the QW material. the QW thick- ness, electron charge. band-gap energy for the QW material, and split-off energy for the QW material. respectively.

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The line shape function L(E.E’) in (3) is taken to be Lorentzian in the following form

/ h \

. , where r, is intraband relaxation time. We are used 7% = 0. I ps. The renormalized band gap with increasing the density of cani- ers in QWs is taken into account as the following

(8) 4e’ AE, =--e iT IC

where K is the static dielechic constant.

0,001- 0 . m - . , * . , . , . , . , . , . I , I , I . .

560 590 600 610 620 630 MO 650 €60 670 660 Wavelength (nm)

Fig. 7. Calculated spectra ofN,ff (a), G (h), G x N , f f and Loss (c) at Ne = 6 x lO”m-?.

Using data for parameters in formulas (4-6, 8) from the re- view by I. Vurgaftman et. al.(’) the gain spectrum was calcu- lated at different e-h density. The spectrum of G per QW at Ne = 6 x 10” cm’ is presented in Fig. 7b and the product of G and N,ff is shown in Fig. 7c with the spectrum of the total losses (Lo.ss)percavitypass. Atthaths=O.O?6and -0.Bxln(R1R2) = 0.09. One can see that the laser action occurs on the short wave side of the gain spectrum. It means that the structure pe- riod is not exactly matched to the gain maximum. It is clear that on conditions of exact tuning, the threshold may be some more decreased.

V. Conclusion

Our results have shown that low threshold lasing requires the position of the QWs to coincide with the antinodes of the cav- ity mode, at the maximum of the gain spectrum due to the QW gound state. We hope that further optimizatioil of the MQW period will decrease the threshold by suitable cavity matching, resulting in increased output power.

Acknowledgements

This work was supported in pan by Russian Academy of Sciences Program ”Low-dimension quantum structures” and Russian Federation Program ”Physics of solid-srate nanostruc- tures”.

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